Conventionally, based on the deviation between the target electric current value and the actual electric current value, which is an actual electric current value of a solenoid, a certain control device may perform a feedback control, such as, a Proportional Integral (PI) control, which is an adjustment process of the electric current flowing in the solenoid by a proportional operation and an integration operation.
In such a PI control, when the proportional gain and/or the integration gain are raised, the actual electric current value tends to overshoot the target electric current value while responsiveness is increased. Such an overshooting is caused because, in view of the deviation between the actual electric current value and the target electric current value, the electric current value is overly accumulated to have an overly-integrated value.
On the other hand, according to the patent document 1 (i.e., Japanese Patent Publication of 2002-182759), a method for performing an adjustment of the target electric current value is described. The described method is capable of changing the target electric current value according to the rise/fall characteristic of the actual electric current value, and is capable of preventing an over-accumulation of the integration value.
However, according to the conventional method, when the target electric current value is adjusted, the adjustment is always the same regardless of the target electric current value or the amount of change of an electric current value. In other words, a convergence time of the target electric current after the adjustment is constant regardless of the amount of change of the target electric current value. Therefore, the adjustment control in response to a small change of the target electric current, which is hard to overshoot, may be delayed according to the conventional method. In other words. If an adjustment control is set to be valid for a large amount of change of the target electric current value, the convergence time of such adjustment is too long for the small amount of change of the target electric current value.
Further, the actual electric current flowing in the solenoid is quick to rise in a low current region and becomes slow to rise when a current region shifts toward a high current region. Therefore, an always-the-same adjustment described above leads to a delayed response especially in the low current region. In other words, if a standard response time is set for the high current region, such a standard response time is too long for the low current region.